Mechanistic framework

The general mechanistic framework outlined in this section must be elaborated by other details to fully describe the mechanisms of the individual electrophilic substitutions. The question of the identity of the active electrophile in each reaction is important. We have discussed the case of nitration, in which, under many circumstances, the electrophile is the nitronium ion. Similar questions arise in most of the other substitution reactions. 

Both stages involve more than one step, and these steps differ in detail among the various carboxylic acid derivatives and for different reaction conditions. This chapter is organized to place the various nucleophilic acyl substitutions into a common mechanistic framework and to point out the ways in which individual classes differ from the rest. 

The high enantioselectivity and broad substrate scope of the HKR are accompanied by an intriguing mechanistic framework involving cooperative catalysis between different catalyst species. Detailed mechanistic investigation into each of these pathways has produced new insights into cooperative catalysis and has resulted in synthetic improvements in the HKR and other ARO reactions [81],... 

Rearrangement to the diphenyline product 3, formally a forbidden [3,5] shift, must take place by a different mechanism in parallel to 2 formation. Previous mechanistic suggestions have attempted to explain the formation of both products within the same mechanistic framework. It is now apparent that 3 is formed by rate-limiting N-N bond fission to give an intermediate from which the product is formed. The nature of this intermediate is not yet known, but it has been suggested16 that it could be a zr-complex. 

More recent mechanistic studies43,44 have confirmed the general mechanistic framework. Acid catalysis is found at acidities up to ca pH 1, then there is an acid region where the rate constant is acid-independent, then at higher acidities acid catalysis occurs again. This is shown in Figure 1. 

Significantly higher values of k5 have been measured for the more polarizable sulfur binding nucleophiles, ca. 102-104 M"1 s-1 (77). The differences in the values of still allow to include most of the studied electrophilic reactions of nitrosyl in a common mechanistic framework, as described by Eqs. (5) and (6). 

This problem was resolved in 1922 when Lindemann and Christiansen proposed their hypothesis of time lags, and this mechanistic framework has been used in all the more sophisticated unimolecular theories. It is also common to the theoretical framework of bimolecular and termolecular reactions. The crucial argument is that molecules which are activated and have acquired the necessary critical minimum energy do not have to react immediately they receive this energy by collision. There is sufficient time after the final activating collision for the molecule to lose its critical energy by being deactivated in another collision, or to react in a unimolecular step. 

As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemical data obtained on hydrogenation of a, /3-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. 

The purpose of this article is to review the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. " A better understanding of the catalytic action of metal complexes... 

These studies suggested a mechanistic framework which could provide a basis for the development of PCDD/F emission control strategies ... 

The intermolecular nature of the hydrogen transfer provided the mechanistic framework that accounted for the results of crossover studies by Aberhart and associates,in which L-[3,3- H2]lysine was shown through analysis by mass spectrometry or NMR to transfer deuterium to L-P-[4,4,5,5- H4]lysine, L-P-[UL- C]lysine, l-P-[2- C]lysine, or L- [3- C]lysine when it was coincubated with the second substrate in the presence of LAM. In these crossover studies, however, the nature of the intermediate hydrogen carrier was not established. 

To meet the challenge of changing olefins technology, future pyrolysis modeling will move toward a more mechanistic framework. The rate of this development will be controlled by the advances made in kinetic theory and by the speed of... 

Catalyst discovery and implementation in transition metal catalysed reactions is even more the combined result of multidisciplinary efforts that join experience on the labile nature of the coordination spheres of complexes with technical efforts to understand the accompanying subtle shifts in their electronic structures by in situ spectroscopy, computational competence in providing reliable mechanistic frameworks, up to engineering efforts to develop miniature devices and microreactors for continuous productions capable of working reproducibly for long periods. Given the sheer number of publications that have appeared in the area over the last 5 years we have restricted our analysis to a few representative examples that appeared since 2000 up to early 2008. 

In terms of coupling these observations of reactive intermediates to a definitive mechanistic framework, the paper of Halpern and Landis plays a central role [33]. Through careful kinetic measurements of hydrogenation as a function of temperature, pressure and catalyst concentration, the soundness of the enamide equilibration/hydrogenation model is established. Rate constants have been derived for the steps defined in Fig. 7 which give self-consistent results for hydro-... 

As outlined above, the Baldwin and Whitehead proposal for the biogenesis of the manzamines has provided a mechanistic framework that can be used to formulate rather detailed step by step hypotheses for the formation of all known 3-alkylpiperidine alkaloids. Figure 3.11 shows an overall scheme for the biogenesis of the 3-alkylpiperidine alkaloids that incorporates elements of all of the individual proposals that have been put forth to date. 

The concept of using zone diagrams to describe behavior within a given mechanistic framework was developed extensively by Saveant and coworkers. Many examples are covered below. The labels given to the zones (e.g., DP, KD, and KP in Figure 12.2.1) are typically derived from their work and are based on their French-language abbreviations. 

The general mechanistic framework outlined in this section can be elaborated by other details to more fully describe the mechanisms of the individual electrophilic substitutions. The question of the identity of the active electrophile in each reaction is important. We have discussed the case of nitration in which, under many circumstances, the electrophile is the nitronium ion. Similar questions about the structure of the active electrophile arise in most of the other substitution processes. Another issue that is important is the ability of the electrophile to select among the alternative positions on a substituted aromatic ring position selectivity). The relative reactivity and selectivity of substituted benzenes toward various electrophiles is important in developing a firm understanding of EAS. The next section considers some of the structure-reactivity relationships that have proven to be informative. 

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